1. Field of the Invention
The present invention relates to the production of filaments from synthetic polymers and, more particularly, to the fabrication of spinnerette plates through which the filaments are spun.
2. Description of the Related Art
The manufacture of synthetic fibers involves the extrusion of polymers, either molten polymers in "melt" spinning, or a polymer solution in "solvent" or "dry" spinning, through a large number of extremely small holes in metal plates. For solvent spinning the hole is typically 30-70 .mu.m across the widest part of the hole. The spinnerette must be relatively thick due to the high viscosity of the polymer solution and the desire for production speeds as high as possible. At the same time, it is desirable that the internal shape and dimensions of all of the holes in the spinnerette have as little variance as possible to produce the highest quality fiber possible. It is also preferable that each spinnerette can be used to produce as wide a range of fiber sizes as possible by modifying other conditions in the fiber production process. Due to the size of such spinnerettes, typically between about 4 and 8 cm in diameter and about 1 to 2 mm thick. It is difficult to obtain spinnerettes with hundreds of precisely shaped holes using conventional techniques.
The conventional method of manufacturing spinnerettes is well known. Examples of methods for manufacturing spinnerettes (also spelled spinnerets and also referred to as jets) are disclosed in U.S. Pat. Nos. 3,362,265; 3,338,085; and 2,730,175. As disclosed in the '265 patent, one or more punching steps are typically performed which result in what will be termed a coined spinnerette. A coined spinnerette typically contains between 40 and 600 protuberances on the side of the spinnerette opposite the punch. These protuberances are ground off and then the final shape of the capillary portion of the hole is formed by a broach. To enable the broach to be properly centered in the hole, the grinding step preferably does not open the hole, so that the broach can be entered using the marks left by a previous punch. Conventional punching machines are able to prevent broaching of the hole during the initial punching step(s). However, the resulting "bumpy" surface of the coined spinnerette opposite the surface on which the punch machinery operates is deformed by the punching process producing a relatively uneven surface on a very small scale, e.g., with variations in the bases of the protuberances in the range of 0.025 m to 0.100 mm. As a result, it is difficult to grind the coined spinnerette to obtain a consistent thickness of the metal at the bottom of each indentation.
Conventionally, the metal protuberances are ground manually using a belt sander. Manually grinding the nonuniform face of a spinnerette requires great dexterity, experience and patience. The process can crudely be described as grind, visually inspect, turn the spinnerette, and repeat. All during this grinding, the metal becomes hotter and hotter. An experienced operator can grind a spinnerette having a large number of holes, e.g., 500 holes, in 45 minutes to an hour. Despite heating of the partially formed spinnerette and the other stresses involved in the process, experienced operators are able to maintain a tolerance of about .+-.25 .mu.m in the remaining thickness of the metal at the bottom of each indentation. It is desirable to reduce this variation to improve the consistency in the size of the spinnerette holes. Previous attempts to automate the process have not led to a significant reduction in the variation of metal thickness.